The present invention relates generally to superconducting materials, and more particularly, to methods of charging superconducting materials.
Many electrical conductors, which can include metallic elements, compounds, and alloys, undergo phase transitions and become superconducting when temperature is reduced below a critical temperature (Tc). Superconducting materials may be characterized by no resistance to the flow of electric current, the tendency to exclude magnetic fields, amongst other interesting magnetic, thermal and electrical properties. Such properties make superconductors potentially useful in a large number of areas including power transmission, digital circuitry, magnets, motors, and many others.
High-temperature superconducting (HTS) materials are generally considered those materials which have a critical temperature of greater than about 20 K. The development of such materials has enabled an increase in the operation temperature of superconducting devices from the liquid helium range (4 K-20 K) to the liquid nitrogen range (60 K-120 K) which has drastically reduced the cost and increased the viability of operating such devices. High-temperature superconducting materials are generally ceramic compounds.
In certain applications, for example when used as magnets, it is necessary to charge superconducting materials. Charging involves inducing a non-decaying electrical current in the superconductor that persists even in the absence of an externally applied magnetic field. The electrical currents flowing in the superconductor generate a magnetic field which may be utilized for a variety of applications. Conventionally, high-temperature superconducting materials may be charged by non-isothermal methods which involve exposing the material to an external magnetic field when the material is not superconducting, decreasing the temperature until the material becomes superconducting (i.e., below the critical temperature), and then removing the externally applied field. In some cases, such conventional charging methods may require that the sample needs to be cooled quickly which may result in large thermal stresses that can cause cracks in the superconducting material. Furthermore, the external magnetic field may need to be applied for relatively long times (i.e., on the order of seconds) which can complicate the design of the charging coils which provide the charging magnetic field. Accordingly, other methods for charging superconducting materials, and in particular near-isothermal methods, may be desirable.
The invention provides methods of charging superconducting materials and, in particular, methods of charging high-temperature superconducting materials. The methods generally involve cooling a superconducting material to a temperature below its critical temperature. Then, an external magnetic field is applied to charge the material at a nearly constant temperature. The external magnetic field first drives the superconducting material to a critical state and then penetrates into the material. When in the critical state, the superconducting material loses all the pinning ability and therefore is in the flux-flow regime. In some embodiments, a first magnetic field may be used to drive the superconducting material to the critical state and then a second magnetic field may be used to penetrate the superconducting material. When the external field or combination of external fields are removed, the magnetic field that has penetrated into the material remains trapped. The charged superconducting material may be used as solenoidal magnets, dipole magnets, or other higher order multipole magnets in many applications.
In one aspect, the invention provides a method of charging a superconducting material. The method includes providing a superconducting material and charging the superconducting material without decreasing the temperature of the superconducting material.
In another aspect, the invention provides a method of charging a superconducting material. The method includes providing a superconducting material, and charging the superconducting material, while the temperature of the superconducting material varies by less than 15 K.
In another aspect, the invention provides a method of charging a superconducting material. The method includes cooling a superconducting material to a temperature below the critical temperature in the absence of an external magnetic field, and charging the superconducting material.
In another aspect, the invention provides a method of charging a superconducting material. The method includes driving a superconducting material into the critical state by one of applying a first external magnetic field, imposing a current in the superconducting material, or a combination of applying a first external magnetic field and imposing a current in the superconducting material. The method further includes applying a second external magnetic field to penetrate into the superconducting material.
Other advantages, aspects, and features of the invention will become apparent from the following detailed description when considered in conjunction with the accompanying drawings.